The invention is directed to a method for compensating non-linearities and time-varying changes of a transfer function acting on an input signal. This input signal is first subjected to a first, adaptive transfer function and thereafter to a second, unknown and varying transfer function to generate the actual output signal. When the second transfer function varies, the first transfer function is updated for compensating these changes. The described method and system are therefore adapted to reduce the errors between an ideally expected output signal and the actually generated output signal.
For example, in the field of mobile communications, there is sometimes the need to linearise a transmitter""s non-linear characteristic, in an adaptive manner, when the characteristic exhibits slow changes caused by temperature, bias, ageing or the like. One of the existing compensating possibilities is the digital predistortion of the input signal before applying same to the transmitter""s power amplifier. Thereby, the error between an ideally expected output signal and the output signal actually generated in response to a current input signal can be minimised. This need for compensation not only occurs in the field of mobile communications but also in other fields where a transfer function varying in an unknown manner is to be compensated by adapting an adaptive transfer function.
When the characteristic is changing, it takes some time until the system has been adapted to the new situation. It is desirable to perform this adaption as swiftly as possible. However, normally, a great number of iterations is necessary until the system has actually been adapted to the new characteristic.
It is therefore an object of the present invention to provide a method and system which are able to swiftly adapt to a changing transfer function and thereby quickly minimise an error between the actual output signal caused by a current input signal, and an ideally expected output signal.
The invention provides a method for compensating deviations of an unknown transfer function from an expected transfer function, and/or for compensating time-varying changes of a transfer function acting on an input signal, for minimising errors of an output signal generated in dependence on the input signal, which input signal is subjected to a first, adaptive transfer function and to a second, varying transfer function to generate the output signal, the first transfer function being updated for compensating deviations of the second transfer function from an expected transfer function and/or for compensating changes of the second transfer function, wherein, when updating one point of the first transfer function for a current input signal value, the first transfer function is also updated for at least one other point corresponding to a different input signal value.
The invention furthermore provides a system for compensating an unknown and/or varying transfer function acting on an input signal, for minimising errors of an output signal generated in dependence on the input signal, which input signal is applied via a first, adaptive compensating means having a first, adaptive transfer function, to a second means having a second, unknown and/or varying transfer function to generate the actual output signal, the first transfer function being updated for correcting deviations of the second transfer function from an expected (ideal, wanted) transfer function by means of a processing means, wherein the processing means is adapted to update, when updating one point of the first transfer function for a current input signal value, the first transfer function also at at least one other point corresponding to a different input signal value.
Due to this multi-point-correction of the first transfer function, the described method and system are able to quickly adapt the first transfer function to changes of the second transfer function and/or to quickly adapt the overall transfer function to the desired one. Thus, the apparatus swiftly converges to the new (or desired) condition, with a drastic reduction of the necessary adaption time. The occurrence of errors between an output signal ideally to be expected for a given input signal, and the actually generated output signal is therefore limited to a very short time interval after a variation (or the first use) of the second transfer function. Furthermore, the error deviations within this time interval are reduced to smaller values.